Digital imaging devices typically have a strobe or flash to add illumination to a scene. In many digital imaging devices, the scene illumination, or brightness, is measured using the photo-sensor in the digital imaging device. The measured scene illumination is used to set the exposure time of the photo-sensor, the aperture of the lens system, and the intensity of the flash, when the flash is required.
Many digital imaging devices have at least two flash settings or modes. In one flash setting or mode, the digital imaging device fires the flash one or more times before the final flash used to capture the image. This flash mode is used to reduce the amount of redeye. Redeye occurs in images of people taken in low light environments while using a flash. The low light environment causes the iris to dilate. When using a flash in a low light environment the strobe light illuminates the blood vessels on the subjects' eye retina, causing the center part of the eye to look red. By firing the flash one or more times before the final flash (sometimes called pre-flashing), the pre-flash illumination causes the iris to close before the final exposure, thus reducing the amount of redeye in the captured image. Unfortunately, the pre-flash consumes energy. That is why most digital imaging devices also have a normal flash mode where the strobe only fires once when the image is captured. This normal mode saves energy but may result in images that contain large amounts of redeye.
Another way to reduce redeye is to increase the distance between the flash and the lens of the imaging device. In general, red-eye occurs when the angel created by drawing a line from the optical center of the lens to the subject's eyes and back to the flash is less than 2.5°. Equation 1 describes this relationship.X=(Y/2)/Tan A  Equation 1Where: X is the camera-to subject distance, Y is the height of the flash above the optical axis of the lens, A is the angle created by the lens axis and the flash to the subject's eyes. To eliminate redeye A should be greater than 2.5°. The two variables we have to work with are camera-to-subject distance (reducing this increases A) and the distance of the flash from the lens (increasing this increases A). Unfortunately, most digital imaging devices are small, with the flash built in. This makes Y small, causing redeye to occur unless the subject is very close to the camera (within 1 to 3 feet).
Currently, users must manually choose between normal and redeye reduction flash modes. There is a need for a digital imaging device that can automatically switch between these flash modes, based upon the photographic situation.